1. Field of the Invention
The present invention relates to the manufacture of a semiconductor device and, more particularly to a fluorine type silicon oxide film used as a layer insulation film and a method of manufacturing the same.
2. Description of the Related Art
Silicon oxide films (SiO.sub.2) formed using plasma enhanced chemical vapor deposition (PE-CVD) have been widely used as insulation films of semiconductor devices. However, the trend toward finer devices at higher levels of integration has given rise to problems associated with burying performance and capacitance between wires and, as a result, silicon oxide films added with fluorine (F) (SiOF) having higher burying performance and lower relative dielectric constants have come into the spotlight as an alternative to SiO.sub.2 described above.
The first method tried to produce an SiOF thin film was to cause plasma discharge of a gas which was a mixture of tetraethylorthosilicate (TEOS), oxygen (O.sub.2) and C.sub.2 F.sub.6 under a low pressure. However, an SiOF thin film produced using this method has a problem in that fluorine atoms therein are very unstable and easily liberated. Further, such fluorine atoms have high moisture absorption characteristics, which can lead to the generation of HF as a result of a reaction with water to corrode a semiconductor and thus can reduce the reliability of the semiconductor.
A technique for improving the quality of such an SiOF thin film is disclosed in articles "Application of SiOF Film Deposition Using TEOS/TEFS/O.sub.2 System to Multi-Level Interconnection Designed with 0.25 .mu.m Rule" by H. Kudo, Fujitsu and "DUAL FREQUENCY PLASMA CVD FLUOROSILICATE GLASS WAFER ABSORPTION AND STABILITY" by M. J. Shapiro, IBM and Toshiba which are incorporated herein as references. According to those articles, it was revealed that the unstableness and moisture absorption characteristics of fluorine atoms in a conventional SiOF film were significantly dependent upon the state of bonding between the fluorine atom and a silicon atom in the film. A major part of the fluorine atoms are diffused in a SiO.sub.2 film at a low concentration in the range from 5 to 15% and are present in a SiOF film in a state expressed by (F--Si(--O--).sub.3) wherein Si atoms and F atoms are bonded in a one-to-one-relationship. However, a very small part of fluorine atoms are in a state expressed by (Si(--F).sub.2 (--O--).sub.2) wherein one Si atom is bonded to two fluorine atoms. The above-described problem is attributable to the fact that fluorine atoms in this state of boding are very unstable from a chemical point of view. This problem becomes more and more serious, the higher the concentration of fluorine atoms in plasma.
The next method tried to mitigate this problem was to use fluorotriethoxysilane (TEFS) or SiF.sub.4 instead of C.sub.2 F.sub.6 described above as a fluorine source gas. Such silicon type fluorine source gases are characterized in that Si--F which is a bond between Si and F atoms in fluorine source gases is not dissociated by plasma. Specifically, a fluorine atom remains in the Si--F bonded state when it is absorbed into the SiOF film. This method makes it possible to reduce the number of free fluorine atoms present in plasma and, as a result, makes it possible to reduce the possibility of the formation of the unstable (Si(--F).sub.2 (--O--).sub.2) state. Although the unstableness of fluorine atoms has been thus significantly mitigated, this method has not been successful in obtaining satisfactory results with respect to the moisture absorption characteristics and quality of a film as a whole.
It is therefore an object of the invention to manufacture a fluorine type silicon oxide film in which fluorine atoms are stable.
It is another object of the invention to manufacture a fluorine type silicon oxide film having low moisture absorption characteristics.
It is a further object of the invention to manufacture a high quality fluorine type silicon oxide film having high compression stress.
It is a still further object of the invention to manufacture a semiconductor device which is manufactured at a low cost with high reliability.